1. Field of the Invention
This invention relates to a light emitting display technology, and more particularly to a current driving method using a pulse width modulation (PWM) technique to display a desired gray level for passive matrix organic light emitting diode (PMOLED) display applications.
2. Description of the Related Art
In modern days, video display devices play an important role in our daily lives. Information and communication routinely are transmitted and then displayed on those devices. Generally, display devices are classified into luminous types and non-luminous types. Luminous type display devices include cathode ray tubes (CRT) and light emitting diodes (LED), while non-luminous type displays include liquid crystal displays (LCD) and the likes.
LCD displays offer the advantages of compactness and power saving, in comparison with conventional CRT displays. However, the drawbacks of long response time, poor contrast and limited viewing angle drive the need for improved technologies. One of the promising display technologies called organic light emitting diode (OLED) display has been developed by Bell et al. of Kodak and described in
An OLED display is an electronic device made by placing a series of organic thin films between two conductors. When electrical currents are applied, a bright light is emitted. OLED displays not only have the favorable characteristics of greater brightness, fast response time, fuller viewing angles and power efficiency over LCD displays, they also are lightweight, compact, and durable with relatively low cost of manufacture. OLED displays are ideal for portable applications. Like LCD devices, OLED displays can be classified into passive matrix mode and active matrix mode. Illumination of an OLED pixel is controlled by a pixel circuit that may include either a source of current or a source of voltage. It is generally recognized that the constant current method provides a greater uniformity of luminance from the arrays of pixels. This is because the dependence of luminance upon current tends to be more uniform while luminance at a given voltage to the various pixels tends to be less uniform.
Passive or conventional matrix driving is generally used for low-resolution OLED displays. However, passively driven resolution is presently limited in the OLED technology to about 100-200 rows for 100 candelas/m2 display brightness levels. Such displays are limited in applications to mobile telephones and mobile video equipment. For example U.S. Pat. No. 6,023,259 to Howard Shin et al. discloses a current driver that provides a passive matrix drive current to an OLED (the entire disclosure of which is herein incorporated by reference).
Referring now to FIG. 1, it illustrates how a conventional PMOLED array works according to the prior art. Control of the luminance of an “on” pixel 100 is commonly achieved by controlling a magnitude of analog voltages that determine whether the column driver voltage exceeds a threshold voltage of the pixel. A traditional manner of changing a displayed image is for a processor to update a memory for a display controller that periodically and individually addresses each of the pixels of the display, and turns them “on” (ON) or “off” (OFF) or to any luminance level in between as required.
Conventionally, in order to create gradation for the tone of each pixel, a so-called pulse width modulation (PWM) technique is used. If the pixel can display 64 gray levels, 64 pulse signals of 0 to 63 with different pulse widths are programmed for driving pixels, and the driving time is proportional to the gray level. However, it is quite complicated to implement the PWM technique for higher resolutions. Therefore, there is still a need to further improve the current driving scheme of the PWM technique for PMOLED display device applications to effectively solve the above-mentioned problems of the prior art.